Emerging third-generation (3G) wireless communication systems support several different kinds of services including voice, high-speed packet data and multimedia services. Further, 3G systems allow users to access several different services simultaneously. To meet the demand for these services, future wireless communication systems will need to provide much higher capacity than second-generation (2G) systems. Greater capacity can be obtained by allocating additional bandwidth, which is unlikely to occur, or by utilizing the allocated bandwidth more efficiently.
WCDMA (Wideband Code Division Multiple Access) is one technology that is expected to help fulfill the demand for 3G services. WCDMA is a multiple access technology for wireless communications over wideband frequency channels. Like narrowband Code Division Multiple Access (CDMA), WCDMA employs spreading codes to spread narrowband signals over the full width of the frequency channel. Each user transmits over a separate code channel and may transmit simultaneously with other users. Signals from multiple users combine during transmission over the communication channel so that the receiver sees the sum of all users' signals that overlap in time and frequency.
Current implementations of WCDMA use a single-user receiver called a RAKE receiver that separately detects signals from each user without considering other users. The RAKE receiver includes a plurality of RAKE fingers matched to a single user's spreading code, but aligned with different time delays to detect different multipaths of the user's signal. The RAKE fingers include a correlator that uses the particular spreading code assigned to the user to despread that user's signal. Signals from all other users are treated as noise. A RAKE combining circuit combines the despread signals output from each RAKE finger to obtain a combined signal with an improved signal to noise ratio (SNR).
While conventional RAKE receivers improve the SNR of the received signal, they do not address interference caused by other users' signals, i.e., multiple access interference (MAI) and intersymbol interference (ISI). MAI and ISI limit the capacity of CDMA systems. MAI is due to cross-correlation between different spreading codes in multipath fading channels. ISI is due to distortion of the transmitted signal that occurs in multipath channels. As the number of users increases, MAI also increases. When the number of users becomes large, conventional single-user receivers may not be able to detect signals from weak users because of high interference levels.
Recently, single-antenna Generalized RAKE (GRAKE) receivers have been developed for better suppressing interference. Interference suppression is achieved by treating ISI and MAI as colored Gaussian noise. The noise correlation across fingers is then exploited by adapting the finger delays and combining weights. In this way, the orthogonality between user signals may be partially restored. GRAKE receivers are described in U.S. Pat. No. 6,363,104, and in U.S. patent application Ser. Nos. 09/344,898 and 09/344,899, which are all incorporated herein by reference.
Multiuser detection (MUD) presents an alternative to single user detection and has been shown to be an effective way to suppress MAI and improve system capacity. In MUD systems, the signals from interfering users are used in the detection of individual user signals. Examples of MUD systems include successive interference cancellation (SIC) and decision feedback (DF). The SIC approach is based on the idea that once a decision has been made about an interfering user's bit, then the interfering signal can be recreated at the receiver using knowledge of the channel and subtracted from the received signal. This process is repeated successively, for one or more other users' signals, and progressively reduces the interference as each of the signals associated with other users is detected. Typically, the strongest signals are detected first and canceled from the received signal, which mitigates the interference for weaker signals.
The DF approach is based on a similar idea, except the subtraction is done on a processed version of the received signal, namely the receiver decision statistics. Furthermore, the subtracted quantity is formed from the previously detected user bits, in a similar manner as decision feedback equalization. While MUD systems are effective in reducing MAI, the complexity of optimal MUD systems increases exponentially with the number of users. Thus, most practical MUD systems use sub-optimal detection systems.
Currently there is a very high level of interest in multiple-input-multiple-output (MIMO) antenna systems for enhancing data rates in third generation (3G) wireless communication systems, particularly for high-speed-downlink-packet-access (HSDPA) in WCDMA and other systems. MIMO has been shown to yield tremendous capacity increases. In typical operating environments, the MIMO channel is frequency-selective, causing inter-symbol interference (ISI) and multiple-access interference (MAI). In addition, self-interference or code reuse interference occurs, even in flat-fading channels, since the spreading codes used in HSDPA are often reused across antennas in order to avoid code limitation problems. In MIMO systems, the challenge is to design a MUD receiver that achieves good performance while maintaining reasonable computational complexity because the processing power of mobile terminals is quite limited and the number of signals to demodulate is large due to multicode and multi-antenna transmission.
Much of the research into MIMO systems has focused on the well-known Vertical-Bell-laboratories-Layered Space-Time (V-BLAST) system where the use of both SIC and DF approaches has been proposed. In V-BLAST systems, interference cancellation using either the SIC or DF approach is based on detected bits before decoding because the different information signals are jointly coded. Recently a promising alternative to V-BLAST has been proposed for use with HSDPA called Per-Antenna-Rate-Control (PARC). This approach is capable of achieving data rates much higher than V-BLAST. The PARC scheme is based on a combined transmit/receive architecture that performs independent coding of the antenna streams at different rates, followed by the application of successive interference cancellation (SIC) and decoding at the receiver. It requires feedback of the per-antenna rates, which are based on the signal-to-interference-plus-noise ratios (SINRs) at each stage of the SIC. With this scheme, it has been shown that the full open-loop capacity of the MIMO flat-fading channel may be achieved, thus offering the potential for very high data rates.